The invention concerns a crystallizing apparatus consisting of a crystallizing conveyor (cooling conveyor) and a precrystallizer, with nucleation being introduced in the precrystallizer in a thin layer and crystallization effected on the cooling conveyor, to which the precrystallized melt is appled in a thick layer.
Crystallizing installations of this type are used for the crystallization of melts, wherein the rate of nucleation is attaining a maximum for a given undercooling. The crystal growth rate is similarly strongly dependent on temperature so that it is desirable in the crystallization of large masses to satisfy the conditions required for optimum nucleation and optimum crystal growth, respectively, so that large masses may be crystallized.
For the purpose, precrystallizers are employed wherein nucleation is initiated, followed by crystallization on endlessly rotating cooling conveyors, upon which the melt is uniformly distributed. Cooling is attained by means of a cooling bath arranged directly under the upper flight of the conveyor or the direct immersion of the conveyor in the cooling bath, so that crystallization takes place while the solidifying melt is on the conveyor, from the end whereof the crystallized product is removed. It is known to use as the precrystallizer a crucible into which the melt is introduced, with the crucible being placed above the onset of the conveyor so that the melt may be poured onto the conveyor through a lower opening. The crucible is maintained at a temperature corresponding to the temperature of the maximum rate of nucleation. To increase the rate of nucleation, a crystal powder is added to the melt in the crucible, which is taken from the crystallizing area and must be ground, representing an additional expense. A further disadvantage of this type of precrystallization is that the temperature must be controlled very accurately to prevent the remelting or the solidification of the melt in the crucible.
In another known configuration, the precrystallizer is arranged in the inlet zone of the melt; it is at a temperature that is optimal for nucleation. With the aid of bean-shaped wipers, seed crystals are transferred from the crystallizing area into this inlet zone of the melt and are able to contribute through their intensive mixing action to a multiplication of the number of nuclei, provided that the temperature is controlled so that the interval of the highest frequency of nuclei is not traversed at an excessive velocity. The fact that seed material must be added to the melt in order to multiply the number of nuclei is a disadvantage in this type of precrystallization. This always leads to an increased equipment outlay and additional seeding material is needed.
It is an object of the invention to minimize problems of the above-described type and to provide a crystallizing apparatus whereby an optimum nucleation rate and number of nuclei may be obtained without the addition of supplemental seed material to a precrystallizer.